Prehydrolysis and digestion of bagasse fibers

ABSTRACT

PULP SUITABLE FOR PRDUCTION OF NEWSPRINT, IS PREPARED FROM SUGARCANE BAGASSE BY CONTROLLED PREHYDROLYSIS UNDER ACID CONDITIONS FOLLOWED BY DIGESTION UNDER ALKALINE CONDITIONS IN THE PREENCE OF AN ALKALI METAL BISULFITE AND THEREAFTER ADDING TO THE PULP, PRIOR TO BLOWDOWN, SUFFICIENT AMOUNTS OF ALKALI METAL SILICATE.

United States Patent U.S. Cl. 162-80 1 Claim ABSTRACT OF THE DISCLOSURE Pulp, suitable for production of newsprint, is prepared from sugarcane bagasse by controlled prehydrolysis under acid conditions followed by digestion under alkaline conditions in the presence of an alkali metal bisulfite and thereafter adding to the pulp, prior to blowdown, sufficient amounts of alkali metal silicate.

This application is a division of my prior copending U.S. Application Ser. No. 149,000 filed June 1, 1971, and now U.S. Pat. No. 3,738,908.

This invention relates to processes for pulping sugarcane bagasse to produce high yield pulp suitable for furnish in the production of newsprint. In particular it relates to processes involving a controlled prehydrolysis of sugarcane bagasse fibers, followed by a controlled alkaline digestion of the prehydrolyzed product, either with an alkali metal silicate alone or together with other alkaline diges tion chemicals, or with other alkaline digestion chemicals alone, followed by treatment with an alkali metal silicate. In the latter case the silicate treatment must be accomplished before or not later than the time that the digested pulp is discharged from the digester to atmospheric pressure. Thus, the silicate treatment may be applied in the digester blow line or at the digester blow valve.

The fibrous fraction of sugarcane bagasse, like other ligno cellulosic materials (e.g., wood, bamboo, etc.), contains as primary constituents cellulose, lignin and hemicelluloses. The latter include both pentosans and hexosans, in amounts averaging about 12 weight percent pentosans (about 90 to 95 weight percent of which is xylan) and about 16 weight percent hexosans (28 weight percent total hemicellulose) based on the total lignocellulosic material in a typical bagasse, on a dry weight basis. The relatively high hemicellulose content of the bagasse binds the cellulose fibers together and renders more difficult the production of suitable pulps, particularly due to the inability to break down the fiber bundles, thus resulting in an inordinate amount of shieve in the paper made from such pulp. As a result it is generally considered impossible to prepare mechanical pulps from bagasse in order to provide furnish for newsprint.

Chemical pulping of bagasse is known and is widely practiced in various countries of the world. The application of the usual chemical pulping processes to bagasse suffers from the disadvantages of considerably reduced yields in view of the amounts of lignin and hemicelluloses removed and of poor brightness and poor opacity in paper produced from the resulting pulp.

The yield from a pulping process is the weight of washed fibers (dry weight basis) recovered after digestion expressed as a percentage of the weight of lignocellulosic material (dry weight basis) originally charged to the pulping process. In all papermaking processes the desire is to achieve the highest possible yield of pulp, suitable for the intended end use, at the lowest possible cost.

There have been a number of earlier attempts to pro- 3,832,278 Patented Aug. 27, 1974 duce pulp from bagasse suitable for furnish in the manufacture of newsprint. Thus, for example, U.S. Pats. 3,013,- 931, 3,013,932 and 3,013,935, each issued Dec. 19, 1961, describe various processes for production of a mechanicaltype pulp from bagasse which can be blended with chemical pulps to produce suitable furnish for newsprint. These processes require the use of blended pulps and also typically require a bleaching or semi-bleaching of one or both of the furnish pulp ingredients. U.S. Pat. 3,081,218 issued Mar. 12, 1963 describes a complicated multistep process for producing newsprint furnish pulp from whole bagasse. This process is obviously relatively expensive to operate and impliedly requires a bleaching step (see, e.g., Col. 1, lines 18 and 19 of U.S. 3,081,218).

In summary it has been observed that prior attempts to produce newsprint furnish from bagasse through use of the classical mild digestion, full digestion, or semi-chemical methods will yield a pulp that will require semibleaching or bleaching. Consequently opacity is lowered necessitating the use of fillers at the papermaking stages. Moreover, yields are reltaively low and the cost of the newsprint is high.

The use of silicates in processes for the digestion of wood is taught in U.S. Pats. 3,013,934 issued Dec. 19, 1961, and 3,147,178 issued Sept. 1, 1964.

It is an object of the present invention to provide processes for producing pulp from bagasse suitable for newsprint furnish without requiring bleaching or semi-bleaching and without the need for fillers or other opacifying agents.

Another object of this invention is to provide a high yield pulping process for producing pulp from bagasse.

Other objects and the attendant advantages of the present invention will be further understood from the following more detailed description.

In its broadest aspects the present invention is based upon the discovery that high yield pulp, having a GE. Brightness of greater than about 55 and thus suitable as such for use as newsprint furnish, can be produced from sugarcane bagasse through a process which comprises, as the essential steps:

(a) Prehydrolyzing depithed bagasse fibers in an aqueous medium having a pH of about 4.5 to about 5.8 in the presence of about 70 to about percent by weight moisture based on the bone-dry weight of the fibers and under autogenous steam pressures at a temperature of from about 340 to about 370 Fahrenheit for a sufficient period of time to remove at least about 40 percent and preferably not more than about 60 percent (by weight) of the xylan in the original fiber;

(b) Thereafter raising the pH of the prehydrolysis medium to at least about 8.5 by addition of alkali metal silicate, alkali metal bisulfite or alkali metal bisulfite plus alkali metal silicate; together with other alkali metal digestion chemicals, if any, needed to provide the requisite pH and digesting the prehydrolyzed fibers in the alkaline medium under autogenous stream pressures at a temperature of from about 340 to about 370 Fahrenheit for an additional period of time sufiicient to provide a pulp having a brightness of at least about 55 and (0) Adding to the pulp, prior to blowdown and where required, sufficient amounts of an alkali metal silicate to provide in the final pulp from about 0.8 to about 1.3 weight percent of said silicate based on the Weight, bonedry basis, of the original bagasse fiber feed.

The resultant pulp is then refined, screened, washed, etc. in the usual manner prior to its use as furnish to the newsprint papermaking machine.

The bagasse fibers used in the practice of the present invention should be as pith-free as is reasonably possible. Suitable bagasse fiber feed materials may be prepared, for example, via the use of the apparatus and/or methods described in US. 'Pat. 3,537,142 issued Nov. 3, 1970 or in my prior copending US. Application Ser. No. 54,580, filed July 13, 1970 and now U.S. Pat. No. 3,688,345. For highest yields it is preferred that the bagasse fiber feed material be one which has been subjected to a two-stage depithing operation, i.e., first dry depithed in accordance with the aforesaid US. Pat. 3,537,142 and then further wet depithed in the presence of at least about 4.5, normally from about 5 to about 10, parts by weight of water per part by weight of fiber (bone-dry basis) in accordance with the said US. Application Ser. No. 54,580 (US. Pat. 3,688,345); the entire disclosure of which is incorporated herein by refrence.

It is essential for the purposes of this invention that the prehydrolysis step be conducted at a carefully controlled pH of from about 4.5 to about 5.8, preferably about 5.2 to about 5.5. At pH below about 4.5 or so the yield from the pulping process is significantly reducedbelow satisfactory economical levels. At higher pH above about 5.9, the desired pre-hydrolysis consumes excessive time, reduces yield and again is economically unattractive. Normally fresh bagasse will have a pH in the range of about 6.0 to 6.5 whereas stored bagasse, as a result of acetic acid produced by fermentation during storage, will have a pH in the range of about 4.5 to 5.0. The pH during the prehydrolysis step can be maintained within the desired limits of 4.5 to 5.8 or preferred limits of 5.2 to 5.5 in various ways, i.e., by mixing fresh and stored bagasse feed in suitable proportions, by controlling the amount of moisture mixed with the bagasse feed or feeds; by addition of minor amounts of pH adjusting chemicals through use of acid prehydrolysis media such as acid white water (pH about 5.5), etc.

The prehydrolysis is conducted in the presence of from about 70 to about 100 weight percent water, based on the bone-dry weight of bagasse fiber feed. Additional water required may be present as liquid water or in the vapor form, i.e., steam, preferably as a liquid water-steam mixture which serves to maintain the appropriate temperature as well as furnish the necessary amounts of water. Liquid water may be blended with the bagasse fiber feed prior to introduction to the prehydrolysis reaction zone or in situ in the zone. Added steam is, of course, intro duced into the prehydrolysis reaction zone.

The prehydrolysis reaction is conducted under autogenous steam pressure at temperatures maintained in the range of from about 340 to about 370 Fahrenheit. Temperature is carefully controlled within the said range in order to achieve the desired result of preferentially removing about 40 to 60 percent of the xylan content of the bagasse fiber feed. This will typically correspond to about 4 to 7 weight percent of the total lignocellulosic material fed, dry Weight basis. The preferred temperature is about 347 Fahrenheit.

The desired prehydrolysis reaction can be achieved, under the stated conditions, in a relatively very short period of from about 5 to about 10 minutes. The xylan removal is not 100% selective, so that some lignin, hexosons and other pentosans will also be removed. However, the xylan removal is at least preferential under the conditions described, so that any reduction in ultimate yield is held to a minimum.

After the prehydrolysis reaction, preferably immediately thereafter, the pH of the prehydrolysis medium is raised to at least about 8.5. The prehydrolyzed fibers are maintained in this alkaline medium and in the same temperature range as that in the prehydrolysis reaction zone for an additional treatment time of about 5 to or minutes, providing an overall total pulping (i.e., prehydrolysis and alkaline digestion) time of about 15 to about minutes. The additional alkaline digestion treatment serves to oxidize, but not remove, lignin components decolored by the initial prehydrolysis reaction.

The final pulp has a total hemicellulose content of about 20 to 22 weight percent comprised predominantly of hexosans together with less than percent of the original xylan. This is an important feature of the present invention. It has been found that removal of the hexosans will not improve the important qualities of the final pulp. Thus, preferential removal of the xylan will provide increased pulp yields and at the same time the best available pulp properties.

The alkaline digestion of the prehydrolyzed bagasse fibers is accomplished by addition of alkali metal silicate, or from about 1 to about 2 percent by weight, based on the weight of bone-dry bagasse fiber feed, of alkali metal bisulfite or a combination of silicate and bisulfite. Other alkaline chemicals (such as sodium sulfite, sodium hydroxide, sodium carbonate, potassium sulfite, potassium hydroxide, potassium carbonate or the like) may be used together with the bisulfite. Where used, the small amounts of alkali metal bisulfite are not overly detrimental to ultimate yields and provide some measure of assistance in improving the brightness of the final pulp.

An essential step in the process of this invention is the addition to the pulp of about 0.8 to about 1,3, preferably about 1.0 weight percent of an alkali metal silicate, based on the bone-dry weight of the initial bagasse fiber feed. This serves to improve the brightness and opacity of newsprint paper produced from the pulp. Silicate in amounts less than about 0.8 weight percent do not improve the brightness and opacity to the desired extent. Amounts greater than about 1.3 weight percent actually begin to degrade the pulp by inducing lignin breakdown and consequent yellowing of the pulp. In addition, ultimate pulp yields will be reduced and there is an undesirable reduction in opacity of newsprint produced from the pulp.

As indicated above the alkali metal silicate or a portion thereof may be added as part or all of alkaline digestion chemicals. Alternately all or part of the required amount of silicate may be added to the pulp separately from the digestion treatment prior to release of the digestion pressure.

In the presently preferred embodiment of the invention the prehydrolyzed bagasse fibers are digested in alkaline medium containing sodium bisulfite, and all of the required alkali silicate is added in the digester blow line or at the digester blow valve, just prior to final completion of the digestion reaction, serving to simultaneously improve the pulp brightness and opacity.

In practice a sodium silicate, such as sodium orthosilicate, sodium sesquisilicate or sodium metasilicate, is the preferred alkali metal silicate. Sodium silicate is available commercially with various ratios of Na O:SiO such as, sodium orthosilicate having a ratio of Na OzsiO of 2: 1, sodium metasilicate having a ratio of Na O:SiO of 1:1, sodium sesquisilicate having a ratio of Na O:SiO of 3:2, as well as other commercial sodium silicate products having various ratios of Na O:SiO such as 1:2, 1:32 etc. However, pulps which exhibit satisfactory color and brightness properties may be obtained by using other alkali metal silicates such as potassiumsilicate and lithium silicate, and, in general, it is contemplated that all of the various forms of the alkali metal silicates such as potassium orthosilicate, potassium metasilicate, lithium orthosilicate, and lithium metasilicate will function in a manner which, in this respect, is similar to sodium orthosilicate or sodium metasilicate. Commercially available potassium silicates having ratios of K O:SiO corresponding to those for sodium silicates are useful as are those having ratios of K O:SiO of 1:2.1, 1:22, 1:25 etc.

The process of this invention is most desirably conducted in a continuous manner, using for this purpose pressure vessels known in the pulp and paper industry as continuous digesters (see, for example, Kirk-Othmers Encyclopedia of Chemical Technology, Second Edition, Volume 16 (1968), pages 700-701, and Rydholm Pulping Processes, Interscience Publishers (1965), pages 343-355); suitably modified, where required, to permit introduction of treatment chemicals at the appropriate point or points in the process.

As previously noted the pulp is further processed in the usual manner, e.g., refined, screened, washed, etc. prior to use as furnish to the production of newsprint or sale to others for such use.

The invention will be further understood from the following specific but non-limiting Example.

EXAMPLE In this example pulp was prepared from bagasse fibers using a commercially available continuous two tub e digester from Pandia, Inc. Water is added to the fiber feed in a wetting tank and the wetted mass is screw fed to the digester. Total average residence time in the digester during the run was about 26 minutes. Digested pulp is sent to an Asplund refiner and thence to a blow tank where pressure is reduced to 0.5 pounds per square inch gauge. The stock is diluted in the blow tank and then pumped through subsequent processing equipment (a cleaner, a centrisorter and three sequentially connected washers) and finally to storage tanks where it is held until fed to a paper making machine.

The digester was adapted for introduction of digestion chemicals and/or other additives at various points along the length of the line of travel through the digester. In this run provision was made for introduction of digestion (cooking) chemicals at points A to /3 from the beginning of the total digester length and for introduction of sodium silicate at the end of the digester, before the digested pulp passed through the Asplund refiner.

In this example the bagasse fiber feed to the digester was wet depithed bagasse prepared in accordance with earlier US. Application Ser. No. 54,580 (US. Pat. No. 3,688,345) referred to above. The fiber had a moisture content of about 50% and contained about 23% lignin and 26% total hemicelluloses on a dry weight basis.

The bagasse fibers were fed to the digester at a rate of about 7.5 tons per hour (bone-dry basis) and were prehydrolyzed at steam pressures of 125 pounds per square inch gauge (about 345 Fahrenheit) in the initial A to /3 portion of the digester with clear white water having a pH of 5.5 in amounts sufiicient to provide about 80 weight percent water in the prehydrolysis zone, based on bone-dry Weight of the fiber feed. The pH of the digestion medium was then raised by introduction of digestion chemicals through the introduction means referred to previously. In this run the digestion chemicals used comprised an aqueous solution having a pH of 11 and containing 46 grams per liter caustic soda (NaOH) 185 grams per liter sodium bisulfite (NAHSO and 19 grams per liter sodium carbonate, which was introduced to the digester at a rate of 24.8 gallons per minute. At the exit end of the digester a 4% aqueous solution of sodium metasilicate was fed into the pulp at a rate of one gallon per minute before the pulp is passed through the Asplund refiner and thence to the blow tank.

The run was conducted for an operating period of about 13 hours under the conditions stated, during which period the yield from the digester was 60-65 percent. The pulp produced had a GE. brightness of 58-60.

In another run conducted under like conditions, except that no silicate was added at any time, the resultant pulp had a GE. brightness not greater than 45-46.

What is claimed is:

1. A method of producing a bagasse derived pulp suitable for use as newsprint furnish wherein the bagasse pulp is produced in a continuous process comprising the following essential steps in sequence:

(a) prehydrolyzing depithed bagasse fibers in a continuous digester at a pH of from about 4.5 to about 5.8 in the presence of from about to about weight percent moisture based on the bone-dry weight of fiber feed and under autogenous steam pressures at temperatures in the range of from about 340 to about 370 Fahrenheit for a sufficient period of time to remove from about 40 percent to about 60 percent of the original xylan content of the bagasse fiber feed;

(b) thereafter raising the pH of the prehydrolysis medium to at least about 8.5 by addition of alkali metal bisulfite alone or together with other alkaline digestion chemicals required to provide the requisite pH and digesting the prehydrolyzed fibers in the alkalized medium and the same continuous digester at the same autogenous steam pressures and temperatures as prevailed for the prehydrolysis step for an additional period of time sufiicient to reduce the total hemicellulose content of the original bagasse fiber feed to the range of from about 20 to about 22 weight percent and provide a pulp G.E. Brightness of at least about 55;

(c) thereafter adding to the pulp, prior to blowdown, sufiicient amounts of an alkali metal silicate to provide a total amount of from about 0.8 to about 1.3 weight percent of said silicate based on the bone-dry weight of the original bagasse fiber feed by the addition of said alkali silicate in the digester blow line or at the digester blow valve just prior to final completion of the digestion;

(d) removing said pulp from said digester; and

(e) refining, screening and washing said pulp, thereby producing a suitable newsprint furnish.

v References Cited UNITED S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R. l6284, 96 

